Fiber-Optic Current and Voltage Sensors for High ... - ABB Group
Invited paper at 16th International Conference on Optical Fiber Sensors, October 13-17, 2003, Nara Japan Technical Digest, pp 752-754.
Fiber-Optic Current and Voltage Sensors for High-Voltage Substations K. Bohnert, P. Gabus, and H. Brändle ABB Switzerland Ltd, Corporate Research, CH-5405 Baden-Dättwil, Switzerland Phone: +41 58 586 8044, Fax: +41 58 586 7358, E-mail: [email protected] Aftab Khan ABB Inc, 100 Distribution Circle, Mount Pleasant, PA 15666, USA
ABSTRACT We report on ABB’s fiber-optic current and voltage transducers and their applications in high-voltage substations. We consider bulk-optics and all-fiber current sensors and voltage sensors that exploit the electro-optic effect in BGO and the piezo-electric effect in quartz.
1. INTRODUCTION In recent years optical current and voltage transducers have reached a high degree of maturity and started to compete with conventional instrument transformers. Fiber-optic transducers are ideally adapted to high-voltage environments as they are highly immune to electro-magnetic interference and there is no galvanic connection between the sensor head on high-voltage and substation electronics. Many problems of their conventional counterparts are inexistent such as magnetic saturation or danger of catastrophic failure. The wide bandwidth of optical sensors is important for fast protection and power quality monitoring. Optical transducers can be easily installed on or integrated into existing substation equipment such as circuit breakers or bushings resulting in significant space savings and reduced installation costs. Furthermore, there is no danger of a contamination of the environment due to loss of oil. In the following, we will consider optical current and voltage sensors that have been developed at ABB and examples of their applications. 2. CURRENT TRANSDUCERS a) Bulk-Optics Current Transducer ABB’s first generation magneto-optic current transducer (MOCT) with more than 10 years of field experience exploits the Faraday effect in a block of fused silica glass with a central aperture for the 1 current conductor . The polarization-rotation of the transmitted light is detected after a single pass around the conductor. The glass body is thermally annealed to eliminate stress-induced birefringence.
Multimode fibers with a 200 µm core diameter guide the light from the LED source to the sensor block and back to the detector. The sensor is mainly used for revenue metering over a primary current range from 3150 A to less than 5 A and reaches accuracy according to IEC class 0.2. For applications requiring particular immunity to shock and vibration a sensor version with two counter-propagating beams has been developed. Subtracting the two corresponding outputs doubles the current-induced signal while any reciprocal modulation due to mechanical effects is largely cancelled. b) All-Fiber Current Transducer More recently, also an all-fiber current sensor has 2 been developed . In comparison to the bulk-optics transducer it offers more flexibility in its design and applications as well as improved performance. Two configurations of the sensor have been investigated and compared. The first one is a Sagnac 3,4 interferometer . The magnetic field of the current produces a differential phase shift between two circular light waves counter-propagating in a coil of sensing fiber. In the second configuration the coil is 5-7 operated in reflection . Here, the phase shift is introduced between two co-propagating left and right circular waves that are reflected at the coil end and then retrace the optical path with swapped polarizations. The circular waves are generated at the coil entrance port(s) from linear waves by means of a short section of elliptical-core fiber acting as a phase retarder. Upon leaving the coil the circular waves are converted back to linear light. The relative current-induced phase shift of the returning linear waves is measured with an appropriately adapted fiber gyroscope module, which also provides the light source. A focus of the development was to achieve insensitivity to temperature and vibration. Commonly, uncontrolled mechanical stress in the sensing fiber is a main cause of drift in scale factor with temperature. The sensing fiber is thermally annealed to remove bend8 induced stress and resides in a thin capillary of fused silica. The capillary prevents any stress from packaging. The coiled capillary is embedded in a soft polymer in a ring-shaped housing (a
photograph is shown in Fig. 1). The temperature -4 dependence of the Faraday effect (0.7x10 /°C) is intrinsically compensated by the temperature dependence of the retarder(s). In the reflective sensor and certain configurations of the Sagnac sensor the scale factor varies with the square of the deviation of the retarders from perfect quarter-wave 6,9 retardation . Hence, the sensor can be designed such that the temperature dependences of the retarder(s) and the Faraday effect just cancel each 2 other . The degree of temperature compensation is controlled in situ during sensor manufacturing. Insensitivity to temperature within
Fiber-Optic Current and Voltage Sensors for High-Voltage Substations K. Bohnert, P. Gabus, and H. Brändle ABB Switzerland Ltd, Corporate Research, CH-5405 Baden-Dättwil, Switzerland Phone: +41 58 586 8044, Fax: +41 58 586 7358, E-mail: [email protected] Aftab Khan ABB Inc, 100 Distribution Circle, Mount Pleasant, PA 15666, USA
ABSTRACT We report on ABB’s fiber-optic current and voltage transducers and their applications in high-voltage substations. We consider bulk-optics and all-fiber current sensors and voltage sensors that exploit the electro-optic effect in BGO and the piezo-electric effect in quartz.
1. INTRODUCTION In recent years optical current and voltage transducers have reached a high degree of maturity and started to compete with conventional instrument transformers. Fiber-optic transducers are ideally adapted to high-voltage environments as they are highly immune to electro-magnetic interference and there is no galvanic connection between the sensor head on high-voltage and substation electronics. Many problems of their conventional counterparts are inexistent such as magnetic saturation or danger of catastrophic failure. The wide bandwidth of optical sensors is important for fast protection and power quality monitoring. Optical transducers can be easily installed on or integrated into existing substation equipment such as circuit breakers or bushings resulting in significant space savings and reduced installation costs. Furthermore, there is no danger of a contamination of the environment due to loss of oil. In the following, we will consider optical current and voltage sensors that have been developed at ABB and examples of their applications. 2. CURRENT TRANSDUCERS a) Bulk-Optics Current Transducer ABB’s first generation magneto-optic current transducer (MOCT) with more than 10 years of field experience exploits the Faraday effect in a block of fused silica glass with a central aperture for the 1 current conductor . The polarization-rotation of the transmitted light is detected after a single pass around the conductor. The glass body is thermally annealed to eliminate stress-induced birefringence.
Multimode fibers with a 200 µm core diameter guide the light from the LED source to the sensor block and back to the detector. The sensor is mainly used for revenue metering over a primary current range from 3150 A to less than 5 A and reaches accuracy according to IEC class 0.2. For applications requiring particular immunity to shock and vibration a sensor version with two counter-propagating beams has been developed. Subtracting the two corresponding outputs doubles the current-induced signal while any reciprocal modulation due to mechanical effects is largely cancelled. b) All-Fiber Current Transducer More recently, also an all-fiber current sensor has 2 been developed . In comparison to the bulk-optics transducer it offers more flexibility in its design and applications as well as improved performance. Two configurations of the sensor have been investigated and compared. The first one is a Sagnac 3,4 interferometer . The magnetic field of the current produces a differential phase shift between two circular light waves counter-propagating in a coil of sensing fiber. In the second configuration the coil is 5-7 operated in reflection . Here, the phase shift is introduced between two co-propagating left and right circular waves that are reflected at the coil end and then retrace the optical path with swapped polarizations. The circular waves are generated at the coil entrance port(s) from linear waves by means of a short section of elliptical-core fiber acting as a phase retarder. Upon leaving the coil the circular waves are converted back to linear light. The relative current-induced phase shift of the returning linear waves is measured with an appropriately adapted fiber gyroscope module, which also provides the light source. A focus of the development was to achieve insensitivity to temperature and vibration. Commonly, uncontrolled mechanical stress in the sensing fiber is a main cause of drift in scale factor with temperature. The sensing fiber is thermally annealed to remove bend8 induced stress and resides in a thin capillary of fused silica. The capillary prevents any stress from packaging. The coiled capillary is embedded in a soft polymer in a ring-shaped housing (a
photograph is shown in Fig. 1). The temperature -4 dependence of the Faraday effect (0.7x10 /°C) is intrinsically compensated by the temperature dependence of the retarder(s). In the reflective sensor and certain configurations of the Sagnac sensor the scale factor varies with the square of the deviation of the retarders from perfect quarter-wave 6,9 retardation . Hence, the sensor can be designed such that the temperature dependences of the retarder(s) and the Faraday effect just cancel each 2 other . The degree of temperature compensation is controlled in situ during sensor manufacturing. Insensitivity to temperature within
